This invention relates generally to power supplies incorporating a DC-to-DC converter and is particularly directed to a DC converter operating at a duty cycle greater than 50% and employing current mode control.
Switchmode power supplies have been rapidly replacing their linear counterparts in the past several years. Significant improvements in efficiency and power density have been the primary advantages of switchmode power supplies. Linear power supplies, however, are still being used in applications where switchmode performance, i.e., the ability to respond to changes in input voltage and output load, is not satisfactory.
The performance of a switchmode power supply is primarily determined by its control section. Current mode control of the switchmode power supply represents a significant improvement over conventional voltage mode control. Whereas conventional voltage mode control relies solely on output voltage feedback, current mode control reacts (turns power switches on/off) to the output inductor current as well, resulting in improved performance. Since the inductor current ramp is a function of the input voltage, single switching cycle correction, which represents optimum power supply operation, to any variation in input voltage is automatically accomplished through inductor current feedback. In current mode control, the inductor current is permitted to vary only as much as needed since the op-amp error voltage V.sub.e limits the inductor current. Conventional voltage mode control allows the inductor current to react at will, resulting in lower reliability and inferior transient response. In current mode control, the inductor current cannot change until the error voltage has changed.
Applying current mode control to a half-bridge converter in the power supply results in a runaway condition wherein one of the capacitors in a voltage divider circuit within the converter completely discharges. This condition is due to different length conduction periods of the two switching elements within the half-bridge converter, across which the two aforementioned capacitors are coupled. This difference in conduction times of the two switching elements may arise from various operating parameters such as propagation delays in the signal paths to each of the switching elements, differences in the respective switching times of the two switching elements, or sub-harmonic oscillations of the voltage-regulating feedback loop employed in voltage mode control. One approach to correcting for this charge versus discharge imbalance condition in the capacitively-coupled half-bridge converter is described in "How To Prevent Runaway Conditions In Half-Bridge Converters Operating With Current-Mode Control," by Herman Neufeld, to be presented at SATECH '86 Conference (Oct. 29, 1986). This approach involves measuring the conduction times of each of the switching elements in determining the relative charge on the capacitors coupled to each of the switching elements. By regulating the conduction times of the switching elements, the discharge rate on each of the capacitors is controlled until both capacitors attain equal voltages. While this approach operates well where the difference in the charge across the two capacitors arises from a difference in the conduction periods of the two switching elements, this approach does not take into account tolerance differences between the two capacitors, individual operating characteristics of the two switching elements, or a mismatch in the voltage across each of the capacitors. Another approach to avoiding this runaway condition which makes use of a balanced winding arrangement in the converter's output transformer is disclosed in Unitrode Power Supply Design Seminar, Oct. 1986, pgs. 1-14 and 1-15.
Inherent to current mode control is instability when the duty cycle of the switching elements exceeds 50%. This instability arises from the fact that the peak inductor current determines the ON time of a switching element and that a small change in the inductor current increases with each switching cycle when the duty cycle is greater than 50%. This may be shown by comparing FIGS. 1a and 1b which illustrate the manner in which a small inductor current perturbation .DELTA.I.sub.o is affected with each switching cycle when the duty cycle is less than 50% and greater than 50% respectively. The conventional way to correct for this instability is to add a ramp to the current sense waveform so that the peak inductor current no longer determines switching element ON time. Thus, to the first current feedback correction signal is added a second compensating ramp correction signal. This approach for improving operation of the current mode half-bridge converter is described in Unitrode Application Note U-97 entitled "Modelling, Analysis And Compensation Of The Current-Mode Controller" by Barney Holland.
Because half-bridge converters are operated with continuous inductor current, the inductor current reflected back into the converter's primary winding appears as a ramp signal superimposed upon a pedestal. If the inductor DC current is large with respect to its AC component, the converter's control circuitry will be susceptible to noise. A small noise spike will result in significant pulse width variation, as illustrated in FIGS. 2a and 2b. Thus, prior art slope compensation attempts to stabilize half-bridge converter operation have met with only limited success because of their high susceptibility to noise.
The present invention is intended to overcome the aforementioned limitations of the prior art by eliminating instabilities in a DC converter power supply incorporating current mode control and operating at greater than 50% duty cycle by detecting and controlling the amount of charge transferred by each switching element in the converter. Although disclosed primarily in terms of use with a half-bridge converter, the present invention is not limited to this arrangement and has application to virtually any type of DC converter employing current mode control.